A hard water sample contains 121 mg of CaCO3 per liter of water. Calculate the mass of Na3Po4 needed to remove all Ca^2+ ions from 2.50 L of the water sample?
Please help! This question is already on here, but I don't understand the part where it says substitute expression into ksp & solve for PO4^3 & the rest?

You have (Ca^2+) from 0.121/molar mass CaCO3.
You know Ksp, plug in Ca^2+ and solve for PO4^3- from the Ksp expression of
Ksp = (Ca^2+)^3(PO4^3-)^2.
That will give you PO4^- in mols/L, convert that to 2.5L, and convert mols to grams by g = mols x molar mass Na3PO4.

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